The invention relates to an improved process of controlling the nitrogen oxide emission from internal combustion engines by retarding spark timing. It is known that retarded spark timing decreases the time for nitrogen to react with oxygen in the combustion chamber at high temperatures and pressures to form nitrogen oxide. A study on 200 cars.sup.(1) for the California Air Resources Board on the effects of disconnection of the vacuum spark advance mechanism of 1966 through 1970 vehicles showed that a 38% reduction of oxides of nitrogen (NO.sub.x) could be attained. FNT (1) California Air Resources Board Staff Report, Testing Vacuum Spark Advance Disconnect (VSAD) under the Applied Research Program, SB848,Nov. 8, 1972.
Automobile manufacturers have used delayed spark advance for the control of NO.sub.x on 1971 and later vehicles to meet California and Federal new car emission standards. California has passed legislation for the mandatory retrofit of used vehicles for devices meeting approval by the California Air Resources Board. To date, two devices using spark timing have been approved providing 41.6% and 44.7% reductions.